Power supplies for electronic devices commonly use a two-stage system, in which incoming utility power, typically 100 to 250 V AC, 50-60 Hz, is first transformed and rectified to produce about 12 V DC power (which may be smoothed to a greater or lesser extent) and is then regulated to produce a sufficiently smooth and stable DC supply for the load device. The regulator may be a switching converter such as a DC-DC “buck converter,” in which the power intake is regulated by rapidly switching on and off an intake transistor or other switch.
These converters are typically regulated to produce a constant output voltage into a resistive load, and thus effectively a constant output power, at least in the short term. If the supply voltage increases, the duty cycle of the switch is reduced, reducing the average current flow into the converter. As measured at frequencies low compared with the switching speed of the converter, this produces the effect of a negative real marginal input impedance. In general, impedance may be purely resistive, purely reactive, or complex with both resistive and reactive parts. If the combined negative input impedance of all the switching converters in a given power supply circuit outweighs the positive real impedances in the part of the circuit from the first-stage transformer/rectifier to the switching converters, the circuit can become unstable. Since the only positive real impedance may be the output impedance of the first-stage transformer/rectifier, that is a very real possibility. It has previously been proposed to stabilize such circuits by including a resistive and/or capacitive load impedance alongside the switching converter to provide additional positive real impedance. However, a predominantly resistive stabilizing load wastes power. A predominantly capacitive stabilizing load requires a substantial capacitance if the stabilizing impedance is to be effective. At low frequencies, typically below a few kHz, the physical size of the capacitor becomes a significant problem for the circuit designer.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.